Nowadays, the energy crisis problem becomes more serious. With increasing environmental consciousness, there are growing demands on renewable energy. For example, solar energy and wind power are the well-known power sources of the renewable energy. In the conventional renewable power generation system, the frequency and the amplitude of the AC power outputted by the power generator are different from the frequency and the amplitude of the AC power network. Consequently, a converting circuit is required to convert the AC power outputted by the power generator. By the converting circuit, the frequency and the amplitude of the AC power outputted by the power generator are identical to those of the AC power of the AC power network. Then, the converted AC power is transmitted to the AC power network through an isolating transformer. As a consequence, the purpose of the grid connected power generation can be achieved.
For example, the wind power conversion system operated in the medium voltage levels (e.g. 1 KV˜10 KV) usually has the following circuitry configurations. The first type is a cascade circuitry configuration of plural power components. The second type is a multi-level (e.g. three-level or more than three-level) circuitry configuration of plural serially-connected power components with lower voltage withstanding capacity (e.g. 1.7 KV). The third type is a multi-level (e.g. two-level or more than two-level) circuitry configuration of plural serially connected power components with higher voltage withstanding capacity (e.g. 3.3 KV, 4.5 KV or 6 KV).
The first type wind power conversion system comprises plural converting circuits (e.g. at least six converting circuits) in cascade connection. For complying with plural independent DC bus lines in the plural converting circuits, the isolating transformer between the AC power network and the wind power conversion system has plural secondary windings (e.g. at least six secondary windings) to be electrically connected with the plural converting circuits. Under this circumstance, the overall cost associated with the factory or instrument is high. Although the second type wind power conversion system comprises plural serially-connected power components (e.g. switch elements) with lower voltage withstanding capacity, more power components are required to produce the multi-level circuitry configuration. Consequently, the method of controlling the power components is more complicated. Moreover, since more power components are employed, the fabricating cost is high. The two-level circuitry configuration of the third type wind power conversion system uses less power components. However, since the power components have higher voltage withstanding capacity, the fabricating cost of the third type wind power conversion system is increased due to the increased cost of using the power components having higher voltage withstanding capacity.
Therefore, there is a need of providing an improved wind power conversion system in order to overcome the above drawbacks.